Lasers
What exactly are lasers? Everyday we hear about them on TV commercials advertising laser eye surgery or hear kids say that they're going to vaporize each other with laser guns, but how do we define a laser? Intro For starters, lets clear up a common misconception. Contrary to popular belief, the word "laser" is not actually a word but an acronym for 'L'ight 'A'mplification by 'S'imulated 'E'mission of 'R'adiation. So what separates a laser from other light sources? Common light sources such as a light bulb or the sun are incoherent, meaning that they emit photons all directions. This light consists of many different wavelengths; the light waves that ARE of the same wavelength, however, tend to be out of phase. To put it simply, incoherent light is very erratic and has no fixed constant form. Unlike light bulbs and the sun, a laser is coherent, meaning that it emits photons in a narrow, concentrated beam. The photons in this near mono-chromatic beam are all the same wavelength and are all in phase. To this day, a laser appears to be the only type of light source that is coherent. So What Are Lasers? This may be hard to explain, considering that much of what occurs with lasers is explained using quantum mechanics and thermodynamics. However, to put it in laymen's terms, lasers and masers are produced by the stimulated emission of photons by atoms in the excited state. If you're still looking at the computer with a confused look on your face, don't worry, it's a hard subject to understand. Although he never built the laser, Einstein predicted what the process would look like. The following steps show this thought process: (The following is quoted from http://www.ryanscreenprinting.com/LaserDefinition.htm; Italics are mine): ''-Atoms are contained in a cavity capped at the ends by two mirrors (one partially transmits light). The atoms are 'pumped' with an infusion of energy.'' ''-The pump energy excites the atoms, putting them in a higher-energy state. Some atoms will spontaneously emit a photon and drop back to their original, unexcited state.'' ''-A spontaneously emitted photon can strike an excited atom, stimulating that atom to emit an identical photon.'' ''-The photons can further stimulate emission from other excited atoms, each of which contributes an identical photon when erverting back to its original state.'' ''-Photons reflect off the mirror, enabling them to stimulate other atoms to emit photons.'' ''-Some photons will leak past the partially transmitting mirror, forming a coherent beam out of the cavity.'' I hope this helps clear up any confusion. If you are still a little confused, try taking a look at the FAQ from http://www.repairfaq.org/sam/laserfaq.htm#faqwil. Hopefully this will answer some of your questions. A Bit of History The groundwork for the laser was discovered in 1917 by Albert Einstein when he first discovered the theory of stimulated emission. Once Einstein developed this theory, however, he left it alone and no one revisited it until after WWII. At the start of the 1950's, there were several scientists attempting to harness energy with the theory of stimulated emission that Einstein had developed several decades before. At this point, scientists were attempting to create the first ever maser ('M'icrowave 'A'mplification by 'S'imulated 'E'mission of 'R'adiation) which amplifies microwaves instead of light waves. Supplementing the work already done in this field by Charles Townes and Alexander Prokhorov, two well known scientists, Theodore Maiman finally invented the first laser in 1960 using a ruby as the medium. Although this first laser was rendered useless because it did not create a strong enough beam, it became the foundation for all of today's modern lasers. Laser Types Although there are thousands of different kinds of lasers, they are all organized within 6 different categories of lasers. The 6 categories contain information on the type of laser, the power output of the laser, the wavelength it emits at, and/or the way it is used in our society: Gas Lasers -The helium-neon laser, the first gas laser to be produced and the first gas laser to produce a continuous beam, is one of the most common lasers used and is often used for educational purposes because it is cheap. It emits at around 543 nm. -A more hardcore gas laser is the carbon dioxide laser which is used in the construction industry for cutting and welding. This laser emits at around 9.6 µm, They also emit up to 100 kW. Chemical Lasers -Being able to achieve high powers, these lasers are fueled by chemical reactions. A Deuterium flouride laser, for example, emits at 3800 nm. Excimer Lasers -Producing ultra-violet light, excimer lasers are the ones used in eye surgeries. To get an example of the range in which these lasers are emitted, F2, an ordinary excimer molecule, emits at 157 nm. Solid-state Lasers -Neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers emit at 1067 nm. Used in the construction industry for the cutting and welding of materials. -Holmium-doped yttrium aluminum garnet (Ho:YAG) lasers at 2097 nm. Used in the medical industry for the elimination of cancers and kidney/gall stones. Semiconductor Lasers -Low powered (5-100 mW) laser diodes are used in everyday items. Things such as CD/DVD players and laser pointers use these kinds of laser diodes. -High powered (up to 10 kW) laser diodes are used in the construction industry. They are used to cut and weld materials such as metal. Dye Lasers -Staying true to its name, dye lasers used organic dyes as a medium. Because there is such a wide variety of organic dyes available, dye lasers can be easily tuned and made to produce extremely quick and short pulses Myths and Legends about Legends Thanks to David's great questions about misconceptions with lasers, I've added this section. Misconceptions: -Laser beams cannot be seen in a vacuum of space. The more particles the laser beam travels through, the more visible it becomes. -The zapping noise lasers make in movies is completely fictional. Laser beams themselves make no sound -Laser beams do not travel at speeds people can actually see (like they do in the movies). Laser beams travel at the speed of light. Resources 1) http://photonotes.org/cgi-bin/entry.pl?id=Incoherentlight 2) http://www.ryanscreenprinting.com/LaserDefinition.htm 3) http://en.wikipedia.org/wiki/Laser 4) http://vcs.abdn.ac.uk/ENGINEERING/lasers/gas.html 5) http://www.lasers.org.uk/laser_welding/briefhistory.htm 6)http://www.repairfaq.org/sam/laserfaq.htm#faqwil Images 1) http://www.vibezproductions.nl/default.asp?task=Laserpointers -The site from which I got the laser pointer picture 2) http://pbskids.org/backyardjungle/discovery/control.php?id=134356&action=detail -The site from which I got the cartoon sun picture 3) http://vcs.abdn.ac.uk/ENGINEERING/lasers/gas.html -The site from which I got the Gas laser picture 4) http://www.gutsehen.de/perfektion/excimer.html -The site from which I got the Excimer laser picture 5) http://www.rp-photonics.com/solid_state_lasers.html -The site from which I got the Solid-state laser picture